In recent years one has become increasingly aware of the impact of human activities on the environment and the negative consequences this may have. Ways to reduce, reuse and recycle resources are becoming more important. In particular, clean water is becoming a scarce commodity. Therefore, various methods and devices for purifying water have been published.
A method for water purification is by capacitive deionisation, using an apparatus provided with a flow through capacitor (FTC) for removal of ions in water. The FTC functions as an electrically regenerable cell for capacitive deionisation. By charging electrodes, ions are removed from an electrolyte and are held in electric double layers at the electrodes. The electrodes can be (partially) electrically regenerated to desorb such previously removed ions without adding chemicals.
The apparatus for removal of ions comprises one or more pairs of spaced apart electrodes (each pair of electrodes comprising a cathode and an anode) and a spacer, separating the electrodes and allowing water to flow between the electrodes. The electrodes are provided with current collectors or backing layers and a high surface area material, such as e.g. carbon, which may be used to store removed ions. The current collectors may be in direct contact with the high surface area material. Current collectors are electrically conductive and transport charge in and out of the electrodes and into the high surface area material.
A charge barrier may be placed adjacent to an electrode of the flow-through capacitor. The term charge barrier refers to a layer of material which is permeable or semi-permeable for ions and is capable of holding an electric charge. Ions with opposite charge as the charge barrier charge can pass the charge barrier material, whereas ions of similar charge as the charge of the charge barrier cannot pass or only partially pass the charge barrier material. Ions of similar charge as the charge barrier material are therefore contained or trapped either in e.g. the electrode compartment and/or in the spacer compartment. The charge barrier is often made from an ion exchange material. A charge barrier may allow an increase in ionic efficiency, which in turn allows energy efficient ion removal.
International patent publication WO 2010/131951 discloses a method for preparing a coated current collector for an electrode for a flow through capacitor. The method comprises preparing an anode coated current collector using water as a solvent. The coated current collectors may be dried in an oven, which may result in a significant reduction in electrode thickness. This way dense electrodes can be obtained by drying the carbon coatings at elevated temperatures.
United States patent publications US 2009/0020430 and US 2010/0170784 disclose the preparation of a coating comprising water, 1-butanol, polyurethane and a carbon mixture. The coating is coated on a carbon paper and dried at room temperature at 80° Celsius.
Some solvent, may be normally retained in the carbon coated current collector after this has been dried in an oven. In fact the retention of solvent in the carbon coated current collector is often beneficial, because it prevents cracking of the carbon coated electrode. Nevertheless, there may be some further drying if the current collectors are stored and/or transported. This further drying may influence the flexibility of the current collectors such that cracks may appear in the carbon coated current collectors for example when they are bended. Cracks may have a negative influence on the working of the carbon coated current collectors and in general cracks should be avoided.